Sheet conveyance apparatus and image forming apparatus

ABSTRACT

A sheet conveyance apparatus can convey a recording paper ( 12 ) in a stable condition irrespective of a kind of the material of the recording paper ( 12 ). The sheet conveyance apparatus conveys a recording sheet ( 12 ) by attaching the recording paper ( 12 ) onto an endless conveyance belt ( 21 ) by applying electric charges onto a surface of the conveyance belt ( 21 ). A charger ( 26, 83 ) charges the surface of the conveyance belt ( 21 ) in a belt-like alternate voltage pattern ( 91 ). A control part ( 73 ) controls a charge width (L) of the alternate voltage pattern ( 91 ) in a direction of conveyance of the recording paper ( 12 ).

TECHNICAL FIELD

The present invention generally relates to image forming apparatusesand, more particularly, to a sheet conveyance apparatus for conveyingsheets using a conveyance belt and an image forming apparatus using sucha sheet conveyance apparatus.

BACKGROUND ART

An inkjet recording apparatus is used as an image recording apparatus(or may be referred to as image forming apparatus) such as a printer, afacsimile or a copy machine. The inkjet recording apparatus records animage on a recording sheet or recording paper by ejecting droplets ofink toward the recording paper from an ink recording head. The recordingsheet is not limited to a regular recording paper but includes an OHPfilm, etc., and means an object onto which droplets of ink or developercan be applied. Hereinafter various kinds of recording sheet may besimply referred to as a recording paper or a sheet. The recording sheetmay also be referred to as a medium to be recorded or a recordingmedium. The inkjet recording apparatus is capable of recording a highdefinition image at high speed. Moreover, the inkjet recording apparatuscan be operated at a low running cost, and has little noise.Additionally, the inkjet recording apparatus has an advantage in that acolor image is easily recordable using multicolor ink.

There is known an inkjet head having a thermal actuator, a shape memoryalloy actuator or an electrostatic actuator, as an energy generatingmeans for discharging ink. The thermal actuator uses a phase changecaused by film boiling of a liquid by an electrothermal conversionelement such as a piezoelectric element. The shape memory alloy actuatoruses a phase change of metal caused by a temperature change. Theelectrostatic actuator uses an electrostatic force to discharge ink.

In the meantime, in an inkjet recording method, since ink is appliedonto a recording paper, the recording paper may expand due to a watercomponent contained in the ink. Such a phenomenon is referred to ascockling. A recording paper may become wavy due to cockling, whichresults in a change in a positional relationship between a nozzle of arecording head and the surface of the recording paper being recorded ata position to a position. If the magnitude of cockling becomes large,the recording paper may be brought into contact with the nozzle of therecording head in the worst case. In such a case, a nozzle surface ofthe recording head may become dirty or the recording paper itself maybecome dirty, which causes deterioration in image quality. Moreover, alanding position of an ink droplet may be shifted due to influence ofcockling, and image quality is deteriorated also in such as case.

In order to solve the above-mentioned problems, an inkjet recordingapparatus performs recording while locating a recording paper on aplaten provided with a recess for absorbing cockling of the recordingpaper. Such a platen is used together with a spur having protrusions onan outer periphery thereof. However, there is a problem in thatscratches are formed on an image on the recording paper by such a spur.

In the conventional inkjet recording apparatus, recording papers areconveyed by rollers. That is, for example, two pairs of rollers arearranged with a printing area located therebetween, and one of the pairsis a combination of a spur and a roller. However, according to such asstructure, an accurate paper feed can be guaranteed only when arecording paper is engaged with both the tow pairs of rollers.

In recent years, there is a demand for increasing a recordable area of arecording paper. Thus, there is an inkjet recording apparatus whichrecords an image on a recording paper so as to acquire a largerecordable area even when the recording paper is in engagement with onlyone of the pairs of rollers, that is, when the paper feed accuracycannot be guaranteed. However, there may be a problem occurs when therecording paper is lifted in a stated where the recording paper is inengagement with only one of the two pairs of rollers. Additionally, thepaper feed accuracy cannot be guaranteed since a sufficient paper feedforce cannot be acquired, which results in a problem of deterioration inimage quality.

In order to pursue an improvement in image quality in an inkjetrecording apparatus, there is a demand for improving an accuracy oflanding position of an ink droplet. Accordingly, it is necessary toimprove a flatness of a recording paper to be recorded. However, asmentioned above, when recording is performed with a regular recordingpaper, fibers in the recording paper may be deformed in a wavy shape dueto a water component contained in ink, which results in a displacementin a landing position of the ink droplet (dot). Thus far, it is foundthat such a deformation of a recording paper appears considerably largeafter more than three minutes have passed after ink droplets landed onthe recording paper.

As disclosed in Japanese Patent Publication No. 2897960 and JapaneseLaid-Open Patent Application No. 7-53081, in order to acquire flatnessof a recording paper, there is suggested an inkjet recording apparatusthat uses an endless charge belt which is electrically chargeable so asto electrostatically attract the recording paper onto the surface of thecharge belt so that the recording paper is conveyed by rotationallymoving the charge belt. Since the recording paper is prevented frombeing separated from the surface of the charge belt due to theelectrostatic attraction force, high-flatness of the recording paper canbe maintained by the charge belt while an inkjet recording operation isperformed.

More specifically, Japanese Patent Publication No. 2897960 discloses apaper conveyance apparatus and an image recording apparatus that chargesthe surface of the charge belt in an alternating pattern by contacting avoltage applying means onto the surface of the charge belt. JapaneseLaid-Open Patent Application No. 7-53081 discloses a printing apparatusthat discharges the charge belt using a discharge belt so as to acquirea stable potential on the surface of the charge belt.

In the meantime, a material of the recording paper used in an imageforming apparatus such as an inkjet recording apparatus is not limitedto one kind since an OHP sheet, etc., may be used other than a regularrecording paper.

In the case where a recording paper is conveyed by beingelectrostatically attached onto a conveyance belt (the same as theconventional charge belt), an electrostatic attraction force is changedsince a rate of dielectric polarization changes due to an influence ofelectric charge on the conveyance belt caused by a surface resistivityof the recording paper to being conveyed.

Accordingly, in the conventional paper conveyance apparatus and imageforming apparatus, when a kind of a recording paper used is changed, adesired electrostatic attraction force may not be acquired, which mayprevents the recording paper from being conveyed in a stable condition.Thus, there is a problem in that deterioration occurs in a conveyancestability and image quality.

DISCLOSURE OF THE INVENTION

It is a general object of the present invention to provide an improvedand useful sheet conveyance apparatus and image forming apparatus inwhich the above-mentioned problems are eliminated.

A more specific object of the present invention is to provide a sheetconveyance apparatus and image forming apparatus that can convey arecording sheet in a stable condition irrespective of a kind of thematerial of the recording sheet.

In order to achieve the above-mentioned objects, there is providedaccording to one aspect of the present invention a sheet conveyanceapparatus for conveying a sheet by attaching the sheet onto an endlessconveyance belt by applying electric charges onto a surface of theconveyance belt, the sheet conveyance apparatus comprising: a chargerthat charges the surface of the conveyance belt in a belt-like alternatevoltage pattern; and a control part that controls a charge width of thealternate voltage pattern in a direction of conveyance of the sheet.

According to the present invention, the control part controls the chargewidth (or charge pitch) of the charge voltage pattern formed on theconveyance belt. Thus, the conveyance belt can be charged with anappropriate charge width (or charge pitch), thereby achieving a stableconveyance of the sheet such as a recording paper.

In the sheet conveyance apparatus according to the present invention,the control part may control the charge width in accordance with a typeof the sheet. The sheet conveyance apparatus may further comprise asheet-type input part that inputs information regarding the type of thesheet to the control part.

Additionally, in the sheet conveyance apparatus, information regardingthe type of the sheet may be given externally. The control part maycontrol the charge width so that the charge width when the sheetcontains a resin is smaller than the charge width when the sheetcontains no resin. The control part may control the charge width so thatthe charge width when a surface resistivity of the sheet is equal to orsmaller than 1×10¹⁰ Ω/□ is set to be substantially equal to or greaterthan 4 mm and substantially equal to or smaller than 30 mm, and thecharge width when a surface resistivity of the sheet is greater than1×10¹⁰ Ω/□ is set to be equal to or greater than 2 mm and equal to orsmaller than 8 mm.

In the sheet conveyance apparatus according to the present invention,the conveyance belt may have a two-layer structure comprising aninsulating layer as an obverse layer and a medium resistance layer as abackside layer. A surface resistivity of the insulating layer may besubstantially equal to or greater than 1×10¹⁰ Ω/□, and a surfaceresistivity of the medium resistance layer may be substantially equal toor smaller than 1×10⁸ Ω/□. Additionally, a thickness of the insulatinglayer may be substantially equal to or smaller than 60 μm, and athickness of the backside layer may be substantially equal to or greaterthan 40 μm. A volume resistivity of a roller with which the conveyancebelt is engaged may be substantially equal to or smaller than 1×10¹⁰Ω·cm.

Additionally, the sheet conveyance apparatus according to the presentinvention may further comprise a discharger that removes or attenuatesthe charges on the surface of the conveyance belt, wherein thedischarger may be located on an obverse side of the conveyance belt anda position out of an area where the sheet is brought into contact withthe conveyance belt.

Further, in the sheet conveyance apparatus according to the presentinvention, the control part may control a charged area of the conveyancebelt when a surface resistivity of the sheet is substantially equal toor greater than 1×10¹² Ω/□ so that at least one of a leading edgeportion and a trailing edge portion of the sheet is attached onto theconveyance belt, the leading edge portion being a range from a leadingedge of the sheet to a position substantially equal to or less than 50mm from the leading edge and the trailing edge portion being a rangefrom a trailing edge of the sheet to a position substantially equal toor less than 100 mm from the trailing edge.

Additionally, in the sheet conveyance apparatus according to the presentinvention, the control part may controls the charge with so that thecharge width is changed in accordance with a distance from a leadingedge of the sheet.

Additionally, there is provided according to another aspect of thepresent invention an image forming apparatus for forming an image on asheet conveyed by a sheet conveyance apparatus conveying a sheet byattaching the sheet onto an endless conveyance belt by applying electriccharges onto a surface of the conveyance belt, the sheet conveyanceapparatus comprising: a charger that charges the surface of theconveyance belt in a belt-like alternate voltage pattern; and a controlpart that controls a charge width of the alternate voltage pattern in adirection of conveyance of the sheet.

Since the image forming apparatus according to the present invention hasthe above-mentioned sheet conveyance apparatus, the conveyance of thesheet is stabilized, which improves an image quality.

In the image forming apparatus according to the present invention, theconveyance belt may be charged before the sheet is fed to the conveyancebelt. A charging operation to the conveyance belt may be stopped whilean image is being formed on the sheet, and the charging operation may beperformed on the conveyance belt when conveying the sheet by a specificdistance.

In the image forming apparatus according to the present invention, apolarity of each charge in the voltage pattern may be changed inaccordance with an amount of movement of the conveyance belt when thesheet is conveyed by the specific distance. The charge width of thevoltage pattern may be an integral multiple of an amount of movement ofthe conveyance belt when conveying the sheet by the specific distance.

Additionally, there is provided according to another aspect of thepresent invention an image forming apparatus for forming an image on asheet conveyed by a sheet conveyance apparatus conveying the sheet byattaching the sheet onto an endless conveyance belt by applying electriccharges onto a surface of the conveyance belt, the sheet conveyanceapparatus comprising: a charger that charges the surface of theconveyance belt in a belt like alternate voltage pattern; a storing partthat stores a relationship between a type of the sheet and a chargewidth of the alternate voltage pattern to be formed on the conveyancebelt; and a control part that controls a charge width of the alternatevoltage pattern in a direction of conveyance of the sheet in accordancewith the type of the sheet based on the relationship stored in thestoring part.

Further, there is provided according to another aspect of the presentinvention a method for conveying a sheet by attaching the sheet onto anendless conveyance belt by applying electric charges onto a surface ofthe conveyance belt, comprising: charging the surface of the conveyancebelt in a belt-like alternate voltage pattern; and controlling a chargewidth of the alternate voltage pattern in a direction of conveyance ofthe sheet in accordance with a type of the sheet based on a relationshipbetween a type of the sheet and a charge width of the alternate voltagepattern to be formed on the conveyance belt.

Other objects, features and advantages of the present invention willbecome more apparent from the following detailed description when readin conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a structural diagram for explaining an inkjet recordingapparatus according to a first embodiment of the present invention.

FIG. 2 is a plan view of the inkjet recording apparatus shown in FIG. 1.

FIG. 3 is a front view of a part of the inkjet recording apparatus shownin FIG. 1.

FIG. 4 is a cross-sectional view of a conveyance belt provided in theinkjet recording apparatus.

FIG. 5 is a block diagram of an entire control part of the inkjetrecording apparatus.

FIG. 6 is an illustration for explaining a charge voltage pattern formedon a conveyance belt;

FIG. 7A is an illustration for explaining charges applied from a chargeroller to the conveyance belt.

FIG. 7B is an illustration for explaining generation of an attractionforce exerted on a recording paper.

FIG. 8 is a flowchart for explaining a control of a charge width of acharge voltage pattern.

FIG. 9 is a time chart for explaining a charge control during a linefeed operation.

FIG. 10 is a time chart for explaining another charge control during aline feed operation.

FIG. 11 is a structural diagram for explaining an inkjet recordingapparatus according to a second embodiment of the present invention.

FIG. 12 is a plan view of a conveyance belt for explaining a chargecontrol performed for a high-resistance object in the inkjet recordingapparatus shown in FIG. 11.

BEST MODE FOR CARRYING OUT THE INVENTION

A description will be given below, with reference to the drawings, ofembodiments of the present invention.

A description will now be given, with reference to FIG. 1 through FIG.3, of an inkjet recording apparatus serving as an image formingapparatus according to a first embodiment of the present invention. FIG.1 is a structural diagram for explaining the inkjet recording apparatusaccording to the first embodiment of the present invention. FIG. 2 is aplan view of the inkjet recording apparatus shown in FIG. 1. FIG. 3 is afront view of a part of the inkjet recording apparatus shown in FIG. 1.

In the inkjet recording apparatus serving as an image forming apparatus,a carriage 3 is supported by a guide rod 1 and a stay (not shown in thefigures) so that the carriage 3 is slidable in a main scanningdirection. The guide rod 1 serves as a guide member mounted between leftand right side plates (not shown in the figures). The carriage 3 ismovable to scan in directions indicated by arrows in FIG. 2 by a mainscanning motor 7 (refer to FIG. 2) through a timing belt.

The carriage 3 is provided with a recording head 4 having four inkjetheads that inject ink droplets in four colors such as yellow (Y), cyan(C), magenta (M) and black (Bk). The recording head 4 is mounted to thecarriage 3 so that a plurality of ink outlet ports are arranged in adirection perpendicular to the main scanning direction and ink dropletsare injected in a downward direction.

In the inkjet head that constitutes the recording head 4, a thermalactuator, a shape memory alloy actuator or an electrostatic actuator canbe used as an energy generating means for discharging ink. The thermalactuator uses a phase change caused by film boiling of a liquid by anelectrothermal conversion element such as a piezoelectric element. Theshape memory alloy actuator uses a phase change of metal caused by atemperature change. The electrostatic actuator uses an electrostaticforce to discharge ink.

Additionally, the carriage 3 is provided with sub tanks 5 of each colorfor supplying ink of each color to the recording head 4 in a replaceablemanner. The ink of each color is supplied to the respective one of thesub tanks 5 from main tanks (not shown in the figures) through inksupply tubes 6, respectively.

The inkjet recording apparatus comprises a semilunar roller (feedroller) 13 and a separation pad 14 together serving as a paper feed partfor feeding recording papers 12 placed on a paper loading part 11 suchas a paper feed tray. The separation pad 14 is formed of a materialhaving a large coefficient of friction, and is urged toward the feedroller 13. The paper feed part separates the recording papers 12 oneafter another and feeds the recording papers 12 from the paper loadingpart 11.

The inkjet recording apparatus further comprises conveyance belt 21, aconveyance guide 22, a counter roller 23, a pressing member 24, anend-pressing roller 25 and a charge roller 26. The conveyance belt 21functions as a conveyance part for conveying the recording papers 12 fedfrom the paper feed part underneath the recording head 4 so as to conveythe recording papers 12 by electrostatically attracting thereto. Theconveyance guide 22 changes the feed direction of the recording papers12 from a vertical direction to a horizontal direction so that therecording papers 12 are brought into contact with the conveyance belt21. The counter roller 23 feeds the recording papers 12 fed from thepaper feed part and guided by the conveyance guide 22 by sandwiching therecording papers 12 between the counter roller 23 and the conveyanceroller 21. The end-pressing roller 25 is urged toward the conveyanceroller 21 by being pressed by the pressing member 24. The charge roller26 is a charging means for charging the surface of the conveyance belt21 so as to generate an electrostatic attraction force.

The conveyance belt 21 is an endless belt, which is engaged with aconveyance roller 27 and a tension roller 28 that provides a tension tothe conveyance roller 21. The conveyance belt 21 is rotated in thedirection indicated by arrow A (belt conveyance direction) by drivingthe conveyance roller 27 by a sub-scanning motor 31 (refer to FIG. 2).

As shown in FIG. 4, the conveyance belt 21 has an obverse layer 21 a anda backside layer (a medium resistance layer or a grounding layer) 21 b.The obverse layer 21 a is formed of a resin material such as a pure ETFEhaving a thickness of about 40 μm. The obverse layer 21 a is not appliedwith a resistance control and forms a recording paper attractionsurface. The backside layer 21 b is formed of a material the same as thematerial of the obverse layer 21 a, and is resistance control by carbonis applied thereto.

If the thickness of the obverse layer (insulating layer) 21 a of theconveyance belt 21 is large, a dielectric constant decreases, whichresults in a decrease in an amount of electric charge retained on theconveyance belt 21 when it is electrically charged. According toexperiments, a desired electrostatic attraction force was acquired bysetting the thickness of the obverse layer 21 a to be equal to orsmaller than 60 micrometers. However, the electrostatic attraction forceof the conveyance belt 21 can be increased by thinning the obverse layer21 a as small as possible in consideration of fluctuation of the filmthickness in a manufacturing process and in a range where the thicknessof the obverse layer 21 a does not become zero even when a flaw isgenerated in the conveyance belt 21 in a practical use.

Although the thickness of the backside layer (a medium resistance layeror a grounding layer) 21 b of the conveyance belt 21 does not give aninfluence directly to the electrostatic action, if a total thickness ofthe conveyance belt is large, a rigidity of the conveyance belt 21increases and it becomes difficult to maintain a flatness of theconveyance belt 21 when being provided in a practical apparatus.However, in order to acquire a necessary strength, the backside layer 21b cannot be made thin too much. According to experiments, it waspreferable to set the thickness of the backside layer 21 b to about 40to 200 μm.

As mentioned above, the conveyance belt 21 has a two-layer structure andprovided with the resistance controlled backside layer 21 b on an entirebackside surface. Accordingly, when a recording paper to be attached isbrought into contact with the conveyance belt 21 after an electriccharge is applied to the obverse layer 21 a which is an insulatinglayer, an electric charge is further supplied to the obverse layer 21 a,thereby increasing the electrostatic attraction force between therecording paper and the conveyance belt 21. If the conveyance belt 21 isformed as a single insulating layer, the electrostatic attraction forceis one half of the case where the conveyance belt 21 has the two-layerstructure. Moreover, if the conveyance belt 21 is a single layer, aposition at which the recording paper initially brought into contactwith the conveyance belt 21 must be a position facing a grounding rollerarranged on an inner side of the conveyance belt 21. However, such alimitation can be omitted by making the conveyance belt 21 with thetwo-layer structure.

In this case, a desired electrostatic force was obtained by using amaterial having a surface resistivity of equal to or greater than 1×10¹⁰Ω/□ for the obverse layer 21 a and using a material having a surfaceresistivity of equal to or smaller than 1×10⁸ Ω/□ for the backside layer21 b.

The conveyance roller 27, which drives the conveyance belt 21, serves asa grounding roller on the charge circuit. The conveyance roller 27 isformed of a material having a high coefficient of friction (high-μ) suchas rubber for the purpose of driving the conveyance belt withoutslippage. Additionally, a certain level of volume resistivity is set tothe conveyance roller 27 to provide an excessive current preventingfunction to the conveyance roller 27 so that a destruction of theconveyance belt 21 is prevented from progressing due to an excessiveelectric current flowing when an electric charging is being applied tothe conveyance belt 21 having the insulating layer destructed by a flawor the like and to prevent generation of a spark or generation ofelectric wave due to an unnecessary electric current. However, if thevolume resistivity is too high, an impedance of the conveyance roller 27on the charge circuit is excessively increased, which causes a problemin that a bias voltage applied to the conveyance belt 21 to form acharge on the conveyance belt 21 is increased. It was foundexperimentally that if the volume resistivity of the conveyance roller27 serving as a grounding roller is equal to or smaller than 1×10¹⁰Ω·cm, the excessive current preventing function can be provided to theconveyance roller 27 at a level at which there is no influence to thebias voltage.

The charge roller 26 is arranged around the conveyance belt 21 so as tobe in contact with the surface of the conveyance roller 21 and rotatedby the rotational movement of the conveyance belt 21. Each end of theaxis of the charge roller 26 is applied with a pressing force of 2.5 N.Moreover, the conveyance roller 27 also serves as a grounding roller,which contacts with the medium resistance layer of the conveyance belt21 so as to ground the conveyance belt 21 through a ground line 29.

Further, the inkjet recording apparatus is provided with a paperejecting part for ejecting the recording paper 12 on which an image isrecorded by the recording head 4. The paper ejecting part comprises aseparating part 41 for separating each recording paper 12 from theconveyance belt 21 and a paper eject tray 42 for accommodating theejected recording papers 12.

A description will be given, with reference to FIG. 5, of a control partof the inkjet recording apparatus. FIG. 5 is a block diagram of theentire control part of the inkjet recording apparatus. The control partcomprises a printer controller 70, a motor driver 81 for driving themain scanning motor 7 and the sub-scanning motor 31, and a head driver82 (constituted by a head drive circuit, a driver IC, etc.) for drivingthe recording head 4 (inkjet head).

The printer controller 70 comprises an interface (hereinafter referredto as I/F) 72, a main control part 73, a RAM 74, a ROM 75, a drivesignal generating circuit 77, an I/F 78 and an I/F 79. The I/F 72receives, through a cable or a network, print data sent by a host sideincluding an information processing apparatus such as a personalcomputer, an image reading apparatus such as a scanner, an imagingapparatus such as a digital camera, etc. The main control part 73includes a CPU, a ROM, a RAM, an I/F, etc., so as to functions as meansfor changing a width of voltage pattern on the conveyance belt 21concerning the present invention. The RAM 74 stores various sets of dataand information. The ROM 75 stores routine or programs for processingvarious sets of data. The drive signal generating circuit 77 generates adrive waveform supplied to the recording head 4. The I/F 78 is aninterface for sending print data, which is developed in dot pattern data(bit map data) and the drive waveform data to a head driver 84. The I/F79 is an interface for sending motor drive data to a motor driver 81 andsending a signal for controlling an output of a high-voltage circuitwhich applies a high voltage (charge voltage) to the charge roller 26.

The main control part 73 is supplied with signals from various sensorsand switches. That is, the main control part 73 is supplied with anoutput of an encoder 34, which is constituted by a photosensor 33 and aslit plate 32 fixed to a shaft 27 a of the conveyance roller 27rotationally driven by the sub-scanning motor 31 as shown in FIG. 2.Additionally, the main control part 73 is supplied with a signal from apaper type designating means (constituted by a switch or menu selectionmeans) provided on an operational panel (not shown in the figures) fordesignating a type of recording paper to be used. Moreover, theinformation which designates a type of the recording paper to be usedcan be provided by a printer driver of a host side. Based on an amountof rotation, that is, an amount of movement of the conveyance roller 27and a designated type of the recording paper to be used, the maincontrol part 73 controls the high-voltage circuit 83 so as to controlthe electric charge of conveyance belt 21.

The RAM 74 is used as various buffers and work memories. The ROM 75stores various sets of information and data such as various controlroutines, font data and graphic functions, various procedures, fixedinformation (table information) of a relationship between a type arecording paper and a width of a charge voltage pattern formed on theconveyance belt 21, etc.

The main control part 73 reads and analyzes print data stored in areception buffer contained in the I/F 72, and stores the obtainedresults of analysis (intermediate code data) in a predetermined area ofthe RAM 74. Then, the main control part 73 produces dot pattern dataused for outputting image data from the stored results of analysis byusing font data stored in the ROM 75, and stores the dot pattern data ina different predetermined area of the ROM 74. It should be noted thatwhen image data is developed on a bit map data by a printer driver ofthe host side and is transferred to the inkjet recording apparatus,received image data of bit map is merely stored in the RAM 74.

When dot pattern data corresponding to one line of the recording head 4is obtained, the main control part 73 sends the dot pattern data of oneline to a head driver 82 as serial data through the I/F 78 insynchronization with a clock signal CLK from an oscillating circuit.Moreover, the main control part 73 sends a latch signal to the headdriver 82 at a predetermined timing.

The drive signal generating circuit 77 is constituted by a waveformgenerating circuit and an amplifier, and includes a ROM (may beconstituted by the ROM 75) storing the pattern data of a drive waveform(drive signal) and a digital-to-analog (D/A) converter which carries outa D/A conversion of the data of the drive waveform read from the ROM.

The head driver 82 comprises: a shift register which inputs clock signaland the serial data which is print data from the main control par 73; alatch circuit which latches a registered value of the shift registeraccording to the latch signal from the main control part 73; a levelconversion circuit (level shifter) which changes a level of an outputvalue of the latch circuit; and an analog switch array (switching means)of which ON/OFF is controlled by the level shifter. The head driver 82selectively supplies to the recording head 4 the necessary drivewaveform contained in the drive waveform by controlling ON/OFF of theanalog switch array.

In the thus-constructed inkjet recording apparatus, the recording papers12 are separated one by one and fed from the paper feed part. Eachrecording paper 12 is guided by the conveyance guide 22, and is fed bybeing pinched between the conveyance belt 21 and each of the counterroller 23 and the end-pressing roller 25.

While each recording paper 12 is being fed, the main control partcontrols the high-voltage circuit 83 (high-voltage source) to apply aplus output and a minus output to the charge roller 26, that is, analternating voltage is applied to the charge roller 26. Thereby, asshown in FIG. 6, the surface (the obverse layer 21 a (insulating layer))of the conveyance belt 21 is charged in a belt-like voltage pattern 91having alternating polarities (hereinafter, referred to as chargevoltage pattern). That is, the conveyance belt 21 is charged plus andminus alternately with a predetermined width (referred to as a chargewidth (or a charge pitch)) in the sub-scanning direction which is arotating direction of the conveyance belt 21.

A description will now be given, with reference to FIGS. 7A and 7B, ofan attraction of a recording paper by an electric charge formed on theconveyance belt 21. As shown in FIG. 7A, a charge voltage pattern 91 isformed on the obverse layer (insulating layer) 21 a of the conveyancebelt 21 by the charge roller 26 applying positive and negative electriccharges at the same potential with a certain width.

In this state, if a recording paper 12 begins to contact with theconveyance belt 21 as shown in FIG. 7B, magnetic lines 90 are generatedon the surface of the conveyance belt 21 from each of the plus electriccharges to an adjacent minus electric charge. Due to the influence ofthe magnetic lines 92, electric charges of the same polarity are inducedon a side of the recording paper 12 opposite to the side where therecording paper 12 is in contact with the conveyance belt 21. Themagnetic line density on the side where the recording paper 12 is incontact with the conveyance belt 21 is higher than that of the sidewhere the recording paper 12 is not in contact with the conveyance belt21. Thus, there is a difference generated in the electric chargesbetween the upper side and the lower side of the recording paper 12,which generates a force (Maxwell stress) toward the conveyance belt 21is exerted on the recording paper 12. Thereby, the recording paper 12 isattracted by the conveyance belt 21, and the recording paper 12 isconveyed by rotational movement of the conveyance belt 21 in thesub-scanning direction.

In this case, the charge width L of the charge voltage pattern 91 formedon the conveyance belt 21 is controlled as follows. First, as shown alsoin FIG. 6, an amount of conveyance (movement) of the conveyance belt 21is obtained (calculated) based on an output of the encoder 34 thatdetects an amount of rotation of the conveyance roller 27. Then, anamount of movement of the conveyance belt 21 is controlled bycontrolling by the main control part 73 the sub-scanning motor 31, whichdrives the conveyance roller 27. Simultaneously, an output of thehigh-voltage circuit (high-voltage source) 83, which applies a highvoltage to the charge roller 26 (charge apparatus) to charge theconveyance belt 21, is controlled to be switched between a positivevoltage (plus) and a negative voltage (minus).

Thus, the recording paper 12 is conveyed by the conveyance belt, whichhas been charged alternately at plus and minus, and stopped at arecording position, and, then, recording of one line is performed byinjecting ink droplets toward the recording paper 12 by operating therecording head 4 while moving the carriage 3 in accordance with imagesignals. Then, when the recording of one line is completed, theconveyance belt 21 is driven so as to move the recording paper 12 by apredetermined distance, and, thereafter, recording of the next line isperformed. Upon receipt of a recording end signal or a signal indicatingthat the trailing edge of the recording paper 12 reached the recordingarea, the recoding operation is ended, and the recording paper 12 isejected onto the paper eject tray 42.

It should be noted that a recovery device for recovering failure ofinjection of the recording head 4 is located at a position out of therecording area on one side in the direction of movement of the carriage3. During standby, the carriage 3 moves to a side near the recoverydevice so as to maintain the recording head 4 in a moist state bycapping the recording head 4 by a capping means, which prevents failureof injection due to dried ink. Moreover, ink droplets irrelevant ofrecording may be injected during recording so as to maintain theviscosity of all ink to maintain a stable injection property.

Moreover, when a failure occurs in injection of ink, the nozzle of therecording head 4 is sealed so as to suction air bubbles together withink by a suctioning means through a tube. Thereby, ink, dirt, etc.,adhering onto the nozzle surface are removed by a cleaning means, and an operation for recovering a failure of injection is performed.

A description will now be given, with reference to FIG. 8 and subsequentdrawings, of a control of a charge voltage pattern formed by the inkjetrecording apparatus. Referring to FIG. 8, the main control part 73retrieves paper type information given by a sheet-type designating means35 or a printer driver on the host side. That is information regardingthe sheet-type of the recording sheet to be conveyed is input from thesheet-type designating means 35, which serves as a sheet-type input partfor inputting the information regarding the sheet-type to the maincontrol part 73, or the information is supplied from an external partsuch as the printer driver on the host side. Moreover, the main controlpart 73 retrieves charge width information corresponding to a designatedpaper type from the table regarding the relationship between paper typesand charge widths stored in the ROM 75.

Then, the main control part 73 controls ON/OFF and plus/minus switchingof the output of the high-voltage circuit 83 so as to switch thepolarity of the voltage applied to the charge roller 26 based on theretrieved charge width information and information regarding an amountof rotation of the conveyance roller 27 detected by the encoder 34 (anamount of movement of the conveyance belt 21). Thereby, the charge widthL of the charge voltage pattern 91 formed on the conveyance belt 21 iscontrolled so as to be equal to the pitch corresponding to the type ofthe recording paper 12 to be conveyed.

The attraction force exerted on the recording paper by the conveyancebelt 21 is influenced by a width of the charged areas on the conveyancebelt 21. That is, a rate of dielectric polarization influenced bycharges on the conveyance belt changes dependent on a surfaceresistivity of an object to be carried, which changes the electrostaticattraction force generated by the conveyance belt. Thus, if the surfaceresistivity of the recording paper is changed due to a change in anamount of water contained in the recording paper or a material formingthe surface of the recording paper, the attraction force exerted on therecording paper with respect to the conveyance belt on which alternatingcharges of plus and minus is changed.

Here, since the surface resistivity of recording papers can beclassified according to types or kinds of the recording papers, theconveyance belt can be charged with an appropriate charge width inaccordance with the recording paper to be conveyed by adjusting(changing) the charge width according to the type or kind of therecording paper. Accordingly, a stable attraction force can be generatedby the conveyance belt, thereby achieving a high conveyance quality andforming an image with high quality.

Moreover, since the surface resistivity of a recording paper fluctuatesdue to changes in the environmental conditions (temperature, humidity),a detecting means (temperature sensor, humidity sensor) for detectingthe environmental conditions may be provided so as to control the chargewidth based on the detected environmental conditions, thereby achievinga more appropriate attraction force.

Here, a description will now be given of a case where a recording paperto be conveyed is a regular paper. Generally, the surface resistivity ofa regular paper is about 1×10⁷ Ω/□. In the case of a regular paper, acharge of a polarity opposite to a polarity of a charge on theconveyance belt 21 is induced on the surface of the recording paper 12,which is brought into contact with the conveyance belt 21. Accordingly,in the electrostatic attraction method according to the presentembodiment, in addition to the above-mentioned electrostatic attractionforce (Maxwell stresses), the charge on the conveyance belt 21 and thecharge induced on the recording paper 12 electrostatically attract eachother, which causes a force that attract the recording paper 12 to theconveyance belt 21. The attraction force caused by the dielectric chargeis stronger than the Maxwell stresses.

The rate of generation of the dielectric charge varies according to thesurface resistivity of a recording paper. If the surface resistivity ishigh (exceeds 1×10¹⁰ Ω/□), the dielectric charge is small and theattraction of the recording paper depends on the above-mentioned Maxwellstresses.

Moreover, the attraction force according to the dielectric charge tendsto increase as the charge width increases within a range where thecharge width L is equal to or greater than 4 mm and equal to or smallerthan 30 mm. On the other hand, the attraction force according to theMaxwell stresses becomes efficient as there are may switching pointsbetween plus and minus charges, that is, as the charge width decreases.

Thus, if the surface resistivity of an object to be conveyed is equal toor smaller than 1×10¹⁰ Ω/□, a regular paper can be conveyed, generally,in a stable condition by setting the charge width equal to or greaterthan 4 mm and equal to or smaller than 30 mm. That is, it is preferableto set the charge width, when conveying a regular paper, to a valueequal to or greater than 4 mm and equal to or smaller than 30 mm.

On the other hand, the surface resistivity of a recording papercontaining a resin such as an OHP sheet (hereinafter, simply referred toas an “OHP” is a value exceeding 1×10¹⁰ Ω/□ unless a solution forcontrolling the resistance is applied onto the contact surface of therecording paper which is brought into contact with the conveyance belt21. Accordingly, the attraction force in the case of attaching an OHP tothe conveyance belt 21 is smaller than that of the case of a recordingpaper having a surface resistivity of 1×10⁷ Ω/□.

If the surface resistivity is high, the attraction force according tothe Maxwell stresses generated in edge portions where the plus and minuscharges on the conveyance belt 21 are switched becomes dominant over theelectrostatic action according to the dielectric polarization. That is,a larger attraction force can be generated for an object to be conveyedhaving a high surface resistivity as there are more switching pointsbetween plus and minus charges, that is, as the charge width decreases.

Thus, if the surface resistivity of an object to be conveyed exceeds1×10¹⁰ Ω/□, a stable conveyance can be achieved by setting the chargewidth within a range equal to or greater than 2 mm and equal to orsmaller than 8 mm. That is, the charge width when conveying an OHP, ispreferably within the range equal to or greater than 2 mm and equal toor smaller than 8 mm. It should be noted that the reason for setting thelower limit of the charge width to 2 mm is that, if it is smaller than 2mm, the charges are cancelled, which is the same as an uncharged stateas mentioned later.

A description will now be given of a relationship between a timing ofstarting a charging operation for the conveyance belt 21 and a timing offeeding the recording paper 12 onto the conveyance belt 21. In theinkjet recording apparatus mentioned above, the recording paper 12 to befed is conveyed by separating it from the pile of recording papers bythe paper feed roller 13. Here, when printing the recording papers 12continuously or printing a plurality of recording papers, there mayhappen a case where a leading edge of one of the recording papers 12 tobe held gradually moves in the direction of conveyance and the leadingedge may be located in the vicinity of the conveyance belt 21.

Moreover, depending on the layout of the image forming apparatus, adistance between the location where the recording papers are piled andthe location where the recording paper 12 is brought into contact withthe conveyance belt 21 may be smaller than a distance between the chargeapparatus (charge roller) on the conveyance belt 21 and the locationwhere the recording paper 12 is brought into contact with the conveyancebelt 21.

In such a case, is becomes impossible to form a charge on the surface ofthe conveyance belt front face, which faces a leading edge of therecording paper.

Thus, before separating and feeding the recording paper 12 so as toperform a print on the recording paper 12, the conveyance belt 21 isdriven in a condition where the paper feed roller 13 is not driven andan output of the high-voltage circuit 83 is applied to the chargeapparatus (charge roller) 26 so as to charge the conveyance belt 21.

As mentioned above, the paper feed roller 13 is driven after formingcharges with a desired charge width on the conveyance belt 21 so as toseparate and feed the recording paper 12, and the recording paper 12 isconveyed to a position directly under the recording head in a statewhere the recording paper 12 is attracted onto the conveyance belt 21,and, then, printing is started.

Thereby, it is prevented that a non-charged area is generated in an areawhere the conveyance belt is in contact with the recording paper 12,which achieves a stable conveyance of the recording papers.

A description will now be given of a relationship between a size of therecording paper 12 and an overall length of the conveyance belt 21. Ifthe conveyance belt 21 is charged while being continuously driven at aconstant speed, the output of the high-voltage circuit 83 applied to thecharge roller 26 can be a constant frequency. Accordingly, thespecification of the high-voltage circuit 83 can be simplified.

Here, if charges are formed beforehand on the conveyance belt 21 and therecording paper 12 is conveyed sorely by the formed charges, and whenthe overall length of the conveyance belt 21 is smaller than the size ofthe recording paper 12, the recording paper 12 is attached to theconveyance belt 21 on a leading edge side but the recoding paper on atrailing side is attached onto the recording paper 12 itself.

In such a case, if the recording paper 12 is once separated from theconveyance belt 21, the charges on the conveyance belt 21 areattenuated. Therefore, in a case where the recording paper 12 having alength longer than the overall length of the conveyance belt 21 isconveyed, the attraction force exerted on a portion near the trailingedge of the recording paper 12 is very small.

Thus, in the present embodiment, when an image formation is performed bythe recording head 4 while the conveyance belt 21 is stopped, thecharging operation to the conveyance belt 21 by the charge roller 26 isalso stopped, but the charging operation is performed when moving theconveyance belt 21 by a desired distance for line feed. Thereby, thecharges on the conveyance belt with respect to the recording paper canalways be maintained constant and a stable attraction force can beobtained.

In the case where the charging operation if performed during a line feed(while the conveyance belt 21 is moved by a desired distance), there isa case where there are a plurality of line feed amounts in some imageforming apparatuses. For example, there is a case where an image isformed with a pixel density larger than a nozzle pitch. That is, theremay be a plurality of line feed amounts in accordance with a pixeldensity of an image to be formed.

On the other hand, the paper attraction force of the electrostaticattraction conveyance method varies depending on the charge width(charge pitch) of the charge voltage pattern of plus and minus charges,and also varies depending on the types of the recording paper, and thereis an optimum charge width for each environment.

Therefore, if a charge voltage of the same polarity is applied duringline feed, it becomes impossible to achieve an optimum charge width.Thus, as shown in FIG. 9, if an amount of movement (line feed amount) ofthe conveyance belt 21 is not an integral multiple of the charge widthis smaller than an amount of a single line feed, the output of thehigh-voltage circuit (high-voltage power source) 83 is switched betweenplus and minus as shown in FIG. 9-(a) while the conveyance belt 21 isdriven (moved by driving the sub-scanning motor 31 as shown in FIG.9-(b).

Then, if the line feed is completed without completely charging at thedesired charge width, the remaining uncharged portion is charged duringa subsequent line feed in the same polarity. Since, in the example ofFIG. 9, the remaining uncharged portion is charged in a portion S1 andan uncharged portion in a portion S2 remains in the line feed concerned,the remaining uncharged portion at S2 is charged in a portion S3 in asubsequent line feed.

As mentioned above, even when the line feed is stopped while forming acharge at a constant width, a desired charge having a desired chargewidth can be formed by switching the polarities of the charge during theline feed. Thus, a stable attraction force can be obtained, whichresults in a stable conveyance property.

Moreover, if it is set to charge the charge belt 21 during line feed(while moving the conveyance belt 21 by a predetermined amount), acharge width may be set so that an amount of movement of the conveyancebelt 21 for a line feed during an image formation is an integralmultiple of the charge width.

Here, the amount of line feed is determined by a pixel density of animage to be formed and a nozzle pitch and a number of nozzles of therecording head 4. Usually, in an image forming apparatus using an inkjethead, a plurality of pixel densities are selectable. If the charge widthis set to 1/n of a greatest common divisor of the line feed amountsprovided by an image forming apparatus, the formation of the charges atthe charge width can always be completed during each line feed as shownin FIG. 10. In this way, there is no need to perform a chargingoperation for a very short time as shown in FIG. 9.

That is, since the switching of polarity is performed during line feedin the example explained with reference to FIG. 9, if the line feedamount is not an integral multiple of the charge width, a charge of thesame polarity is applied over two line feeds. Then, as shown in FIG. 9,it is possible that a charge voltage is applied for an extremely shorttime (a portion indicated by S4 in the figure), and an amount of chargecharged at that time may not satisfy a desired amount.

That is, according to such a charging operation for an extremely shorttime, a desired charge potential may not be formed on the conveyancebelt 21 through the charge roller 26 even if the output of thehigh-voltage circuit 83 is raised to a desired voltage. Accordingly, thecharge potential may be lower than a desired level, and the chargingoperation may be ended before the potential of the portion to be chargedis not sufficiently raised.

On the other hand, if the charge width is set so that the line feedamount is an integral multiple of the charge width as shown in FIG. 10,the charge of the same polarity is not applied over two line feeds andthe charge area is charged with a desired charge width at once. Thus,the amount of charge on the conveyance belt can be a desired amount,which achieves stabilization of the potential level.

A description will now be given, with reference to FIG. 11, of an inkjetrecording apparatus as an image forming apparatus according to a secondembodiment of the present invention. In this embodiment, a dischargedevice such as a discharge brush for removing a residual charge on theconveyance belt 21. It should be noted that the discharge device 38 iselectrically connected to a housing of the inkjet recording apparatus orthe like.

Therefore, electric charges after the separation of a recording paper isremoves by the discharge device 38 when the conveyance belt 21 is movedwhile forming electric charges of plus and minus on the conveyance belt21. Thereby, when charging the conveyance belt 21 by the charge roller26, there is no charge formed on the conveyance belt, which stabilizesthe charge and improves the durability of the conveyance belt 21.

That is, if charges of plus and minus (charge voltage pattern) arealternately formed on the entire surface of the conveyance belt 21, apotential of the surface of the conveyance belt 21 in an area where theconveyance belt 21 is in contact with the recording paper 12 decreaseswhen the separation of the recording paper 12 is performed.

However, a charge remains on the surface of the conveyance belt 21 in anarea which is not brought into contact with the recording paper 12 sincethe area is located between the adjacent recording papers 12. If acharge having a polarity opposite to the polarity of the remainingcharge should be formed in that area, there is a potential difference aslarge as twice the voltage to be applied.

Generally, there is a small destruction of an insulating layer due toattack of the charge on the surface of the conveyance belt. If thecharge is applied with a potential difference as large as twice thevoltage normally applied as mentioned above, the magnitude of attackincreases, which may result in an adverse effect to the durability ofthe conveyance belt 21. Thus, by providing the discharge device 38 so asto set a state where no electric charge is formed on the conveyance belt21 when a charging operation is performed, there is no need to apply acharge with a potential difference of about twice the normally appliedvoltage, thereby improving the durability of the conveyance belt 21.

Moreover, when conveying the recording paper 12, there is a case, asmentioned below, where it is not desirable to form a charge in a portionof the conveyance belt corresponding to a specific portion of therecording paper 12. In such a case, a control can be performed so that acharge is formed only on specific portions of the conveyance belt byusing a discharge mechanism having the discharge device.

A description will now be given, with reference to FIG. 12, of a controlof a charged area when conveying a recording paper, which is ahigh-resistance object having a high surface resistivity (exceeding1×10¹² Ω/□) in the inkjet recording apparatus. In this case, a leadingedge portion LE of the recording paper conveyed is detected, and acharge is applied only to a portion corresponding to a leading edgeportion (equal to or less than 50 mm) and a portion corresponding to atrailing edge portion TE (equal to or less than 100 mm) so as to form anuncharged area 93 therebetween.

With this apparatus, since the discharge device 38 for removing a chargeon the conveyance belt 21 is provided, a charge is never formed in theuncharged area 93 on the conveyance belt 21.

Therefore, when the recording paper 12, which is a high-resistanceobject, is conveyed by the conveyance belt 21, the leading edge portionLE and a trailing edge portion TE of the recording paper areelectrostatically attached onto the conveyance belt 21. In many cases,the recording paper 12, which is a high-resistance object to beconveyed, contains a resin, and the balance of the recording paper 12will not be off due to an image formation by ink. Thus, there is lessnecessity of electrostatically attaching the recording paper 12 onto theconveyance belt 21. The leading edge portion LE is electrostaticallyattached onto the conveyance belt 21 since is it necessary to form animage in a state where the position of the recording paper is notmaintained well. In addition to the above-mentioned reason, the trailingedge portion is electrostatically attached onto the conveyance belt 21since it is required to surely eject the recording paper into the papereject tray 42 after the image recording.

As mentioned above, when conveying the recording paper, which is ahigh-resistance object to be conveyed, only the leading and trailingedge portions LE and TE are electrostatically attracted by theconveyance belt. Accordingly, also when the recording paper is separatedfrom the conveyance belt, an electric charge due to a release dischargemay be generated on the surface of the recording paper at the leadingand trailing edge portions. However, such a recording paper having anelectric charge on the leading and trailing edge portions can be stackedwithout electrostatic attachment with a high-resistance objectpreviously ejected from the apparatus since the charged area due to therelease discharge.

That is, when a recording paper having a high surface resistivity(1×10¹² Ω/□) is conveyed by electrostatically attaching to theconveyance belt, the release discharge is generated between the chargeson the surface of the recording paper and the charges on the conveyancebelt when the recording paper is separated from the conveyance beltsince the charges on the surface of the recording paper do not easilymoves. If such a release discharge occurs, the portions of the recordingpaper, which has been in contact with the conveyance belt 21, arecharged. As a result, when the recording paper is separated and ejectedonto the paper eject tray, the ejected recording paper may beelectrostatically attached to recording papers previously ejected andstored in the paper eject tray, which may give a bad influence such ascausing eject of the recording papers stored in the paper eject tray orcausing a conveyance resistance to a recording paper being conveyed.

Then, when the recording paper, which is a high-resistance object, isconveyed, portions of the conveyance belt corresponding to the leadingand trailing edge portions are charged so that only the leading andtrailing edge portions LE and TE of the recording paper are attachedonto the conveyance belt. That is, portions other than the leading andtrailing edge portions are not charged so as to decrease the chargedarea generated by a release discharge to a level causing no problem inthe conveyance. Thereby, the electrostatic attraction between theejected recording paper and recording papers previously ejected onto thepaper eject tray is prevented, thereby improving stackability andconveyance property.

Furthermore, by setting the charge width of the charge voltage patternformed on the conveyance belt to be equal to or smaller than 2 mm, acondition can be established in which no charge is formed on theconveyance belt. Although it is possible to cancel a charge formed onthe conveyance belt by the next charge of the opposite polarity, thischarge may act to discharge also the conveyance belt having a residualcharge thereon.

Although the above-mentioned charge control as shown in FIG. 12 isperformed when conveying the high-resistance object, there may exist aresidual charge of a certain level in the uncharged area 93 depending onthe effect of the discharge device. In such a case, if a high frequencyvoltage with a pitch of 2 mm or less is applied on to the conveyancebelt, the conveyance belt can be forcibly discharged so as to form anuncharged area without using the discharge device.

That is, as shown in FIG. 12, as for the charge control operation, thecharged area of the conveyance belt 21 corresponding to the leading edgeportion LE of the recording paper 12 is charged at a normal chargewidth; a high-frequency voltage is applied at a timing of charging theuncharged area; and the charged area of the conveyance belt 21corresponding to the trailing edge portion TE is charged at the normalcharge width. Thus, by controlling the charge width in accordance with adistance from the leading edge of the recording paper, the chargevoltage pattern 91 can be formed only on a desired portion of therecording paper.

Although the present invention is applied to the inkjet recordingapparatus of a serial type (shuttle type) in which a carriage is scannedin the above-mentioned embodiments, the present invention is alsoapplicable to a line-type inkjet recording apparatus having a line-typehead.

Moreover, the image forming apparatus according to the present inventionis applicable to a facsimile apparatus, a copy apparatus and aprinter/fax/copy combination machine besides an inkjet printer.Furthermore, the present invention is applicable to an apparatus thatinjects a liquid other than ink, such as, for example a resist or a DNAsample in a medical field.

The present invention is not limited to the specifically disclosedembodiments, and variations and modifications may be made withoutdeparting from the scope of the present invention.

1. A sheet conveyance apparatus for conveying a sheet by attaching thesheet onto an endless conveyance belt by applying electric charges ontoa surface of the conveyance belt, the sheet conveyance apparatuscomprising: a charger that charges the surface of said conveyance beltin a belt-like alternate voltage pattern; and a control part thatcontrols a charge width of the alternate voltage pattern in a directionof conveyance of the sheet.
 2. The sheet conveyance apparatus as claimedin claim 1, wherein said control part controls the charge width inaccordance with a type of said sheet.
 3. The sheet conveyance apparatusas claimed in claim 2, further comprising a sheet-type input part thatinputs information regarding the type of said sheet to said controlpart.
 4. The sheet conveyance apparatus as claimed in claim 2, whereininformation regarding the type of said sheet is given externally.
 5. Thesheet conveyance apparatus as claimed in claim 2, wherein said controlpart controls the charge width so that the charge width when the sheetcontains a resin is smaller than the charge width when the sheetcontains no resin.
 6. The sheet conveyance apparatus as claimed in claim2, wherein said control part controls the charge width so that thecharge width when a surface resistivity of the sheet is equal to orsmaller than 1×10¹⁰ Ω/□ is set to be substantially equal to or greaterthan 4 mm and equal to or smaller than 30 mm, and the charge width whena surface resistivity of the sheet is greater than 1×10¹⁰ Ω/□ is set tobe substantially equal to or greater than 2 mm and substantially equalto or smaller than 8 mm.
 7. The sheet conveyance apparatus as claimed inclaim 1, wherein said conveyance belt has a two-layer structurecomprising an insulating layer as an obverse layer and a mediumresistance layer as a backside layer.
 8. The sheet conveyance apparatusas claimed in claim 7, wherein a surface resistivity of said insulatinglayer is substantially equal to or greater than 1×10¹⁰ Ω/□, and asurface resistivity of said medium resistance layer is substantiallyequal to or smaller than 1×10⁸ Ω/□.
 9. The sheet conveyance apparatus asclaimed in claim 7, wherein a thickness of said insulating layer issubstantially equal to or smaller than 60 μm, and a thickness of saidbackside layer is substantially equal to or greater than 40 μm.
 10. Thesheet conveyance apparatus as claimed in claim 7, wherein a volumeresistivity of a roller with which said conveyance belt is engaged issubstantially equal to or smaller than 1×10¹⁰ Ω·cm.
 11. The sheetconveyance apparatus as claimed in claim 1, further comprising adischarger that removes or attenuates the charges on the surface of saidconveyance belt, wherein said discharger is located on an obverse sideof said conveyance belt and a position out of an area where said sheetis brought into contact with said conveyance belt.
 12. The sheetconveyance apparatus as claimed in claim 1, wherein said control partcontrols a charged area of said conveyance belt when a surfaceresistivity of said sheet is substantially equal to or greater than1×10¹² Ω/□ so that at least one of a leading edge portion and a trailingedge portion of said sheet is attached onto said conveyance belt, theleading edge portion being a range from a leading edge of said sheet toa position substantially equal to or less than 50 mm from the leadingedge and the trailing edge portion being a range from a trailing edge ofsaid sheet to a position substantially equal to or less than 100 mm fromthe trailing edge.
 13. The sheet conveyance apparatus as claimed inclaim 1, wherein said control part controls the charge with so that thecharge width is changed in accordance with a distance from a leadingedge of said sheet.
 14. An image forming apparatus for forming an imageon a sheet conveyed by a sheet conveyance apparatus conveying a sheet byattaching the sheet onto an endless conveyance belt by applying electriccharges onto a surface of the conveyance belt, the sheet conveyanceapparatus comprising: a charger that charges the surface of saidconveyance belt in a belt-like alternate voltage pattern; and a controlpart that controls a charge width of the alternate voltage pattern in adirection of conveyance of the sheet.
 15. The image forming apparatus asclaimed in claim 14, wherein said conveyance belt is charged before saidsheet is fed to said conveyance belt.
 16. The image forming apparatus asclaimed in claim 14, wherein a charging operation to said conveyancebelt is stopped while an image is being formed on said sheet, and thecharging operation is performed on said conveyance belt when conveyingsaid sheet by a specific distance.
 17. The image forming apparatus asclaimed in claim 16, wherein a polarity of each charge in said voltagepattern is changed in accordance with an amount of movement of saidconveyance belt when said sheet is conveyed by the specific distance.18. The image forming apparatus as claimed in claim 16, wherein thecharge width of said voltage pattern is an integral multiple of anamount of movement of said conveyance belt when conveying said sheet bythe specific distance.
 19. An image forming apparatus for forming animage on a sheet conveyed by a sheet conveyance apparatus conveying thesheet by attaching the sheet onto an endless conveyance belt by applyingelectric charges onto a surface of the conveyance belt, the sheetconveyance apparatus comprising: a charger that charges the surface ofsaid conveyance belt in a belt like alternate voltage pattern; a storingpart that stores a relationship between a type of said sheet and acharge width of the alternate voltage pattern to be formed on theconveyance belt; and a control part that controls a charge width of thealternate voltage pattern in a direction of conveyance of the sheet inaccordance with the type of said sheet based on said relationship storedin said storing part.
 20. A method for conveying a sheet by attachingthe sheet onto an endless conveyance belt by applying electric chargesonto a surface of the conveyance belt, comprising: charging the surfaceof said conveyance belt in a belt-like alternate voltage pattern; andcontrolling a charge width of the alternate voltage pattern in adirection of conveyance of the sheet in accordance with a type of saidsheet based on a relationship between a type of said sheet and a chargewidth of the alternate voltage pattern to be formed on the conveyancebelt.